The present invention generally relates to blending gasoline and butane at gasoline tank farms. More specifically, it allows for simple and accurate blending of gasoline and butane at the point of distribution, based upon the volatility of the gasoline before blending, and a prescribed level of volatility after blending.
Gasoline distribution systems typically rely upon pipelines to deliver gasoline from refineries to tank farms. At the tank farms gasoline is stored until it is dispensed to gasoline tanker trucks. The tanker trucks deliver the gasoline to retail gasoline stations and other dispensing outlets where the gasoline is eventually delivered to an automobile, truck, or other vehicle.
A significant physical property of gasoline is its volatility, or its ability to combust. There are two principle methods for assessing the volatility of gasoline: (1) measuring the vapor-liquid ratio, and (2) measuring the vapor pressure. The Reid method is the standard test for measuring the vapor pressure of petroleum products. Reid vapor pressure (sometimes xe2x80x9cRVPxe2x80x9d) is related to true vapor pressure, but is a more accurate assessment for petroleum products because it considers sample vaporization as well as the presence of water vapor and air in the measuring chamber. The Reid vapor pressure of gasoline affects the ease with which gasoline is combusted, and can have a significant impact on the ease with which an automobile engine is started, especially during colder seasons when the temperature of gasoline and its corresponding volatility decreases.
In order to keep cars performing at a consistent level year-round, gasoline marketers blend agents with gasoline that increase the Reid vapor pressure and volatility of the gasoline. Butane is commonly added as a RVP modifying agent during colder months because it is more volatile than gasoline. It is also added to reduce the cost of gasoline, because it is generally less expensive than gasoline bought at the wholesale level.
The United States Environmental Protection Agency (EPA) is concerned with gasoline vapor pressure because gasoline vapor emissions from automobiles are a major component of VOC""s (volatile organic compounds) in the atmosphere. Gasoline vapor emissions are of particular concern during the warmer months when gasoline is more volatile. Accordingly, the EPA has promulgated a number of regulations that govern the volatility of gasoline, and how much butane can be blended with gasoline during May 1 through September 15. For each occasion that butane is blended with gasoline during this time period, the EPA requires that the mixture be certified to ensure that it is within the volatility guidelines.
Butane has historically been blended with gasoline at several points in the gasoline distribution chain. The first opportunity to blend butane with gasoline is at the refinery, before pipelines transport the gasoline to tank farms. Refineries often add butane at the trunk line in response to changes in Reid vapor pressure demand. This process is imprecise, however, because the blended gasoline is subsequently mixed in the pipeline with other sources of gasoline of varying Reid vapor pressure. Moreover, because gasoline pipelines serve multiple regions that have variable RVP requirements, the refinery can only modify the gasoline to the lowest maximum RVP allowed by the EPA across the various regions served by the pipeline.
Butane is also added to gasoline while it is transported in the pipeline, after consolidation of various trunk lines from refineries. Typically, butane will be added to a certain volume of gasoline with a constant volatility. However, it is difficult to perform blending in the pipeline with any measure of precision. The rates of flow within the pipelines and the Reid vapor pressure of a certain volume of gasoline within the pipeline vary considerably. An additional difficulty is that the pipeline must be physically breached in at least two locations to sample the gasoline, and to add butane to the flow of gasoline. Breaching a high volume gasoline pipeline carries with it large risks that most pipeline operations would prefer to avoid.
The third point of blending is at the tank farm. When delivery of gasoline is made to a large storage tank, the RVP of the tank is measured, and sufficient butane is added to the tank to attain a desired RVP. Because tanks farms generally service a smaller area than an entire pipeline, blending at the tank farm can be more narrowly tailored to the RVP requirements of a particular region. Blending in tanks at the tank farm is performed with existing valves in tanks to introduce the butane, and thus does not entail risks similar to butane blending during gasoline transport through a pipeline.
However, blending butane at tank farms is not without its complications. Each time that gasoline is introduced to a tank, the RVP must again be measured, and butane must be added to the, tank to attain a desired RVP. Often, gasoline will be dispensed to several tanker trucks before the butane can be blended, thus losing the opportunity to blend butane in those shipments. Moreover, blending butane in tanks is labor intensive and imprecise. Because the RVP of gasoline varies within a tank, several measurements must be taken throughout the tank, and considerable stirring must occur to maximize homogeneity.
All of the foregoing methods suffer from a substantial degree of impression. In view of this impression, gasoline suppliers are unable to maximize the amount of butane blended with gasoline. In particular, during summer months, government regulations currently require recertifying of gasoline each time butane is blended. The cost of this additional certification vitiates gains that could be achieved with additional blending at the tank farm.
Several methods have been attempted to improve the precision of butane blending and the predictability of Reid vapor pressure in the final product. The Grabner unit is a substantial advance in this respect. The Grabner unit (manufactured by Grabner Instruments) is a-measuring device capable of providing Reid vapor pressure and liquid-vapor ratio data for a gasoline sample typically within 6-11 minutes of introducing the sample to the unit. It has been employed at some refineries to consistently measure the volatility of gasoline, and to blend butane with the gasoline based upon an allowable RVP for the gasoline. There are also other commercially available instruments for assessing the volatility of gasoline.
Although the Grabner unit can provide more accurate assessments of gasoline volatility at the refinery, a need exists for precise measurements at the final distribution point, which is the tank farm. As explained above, the current method of taking several measurements from a large gasoline tank at the tank farm is labor intensive and must be done repeatedly because of new gasoline being introduced into the tank. A further need exists in that tank farm distributors must constantly adjust their butane blending throughout the year in response to changes in temperature and concomitant changes in EPA allowable RVP levels. Finally, a need exists for the ability to blend butane with reformulated gasoline more accurately.
The present invention is a system and method for blending butane with gasoline at the tank farm, immediately before the gasoline is dispensed to a tanker truck. The blending occurs downstream of the gasoline and butane storage tanks on the tank farm, after the gasoline and butane are drawn from their storage tanks for dispensing into a tanker truck, but before the gasoline is actually dispensed to the tanker truck at the rack. The apparatus for blending the utane and gasoline is any conventional Y-type or T-type juncture capable of joining two fluid flows into one. The ratio of gasoline and butane blended by the blending apparatus can be varied to achieve any desired vapor pressure or vapor/liquid ratio in the gasoline dispensed to the tanker truck.
The blending apparatus is preferably under the continuous control of a process control unit, which can vary the ratio at which gasoline and butane are blended to attain a desired vapor pressure or vapor/liquid ratio. The process control unit determines the blending ratio based upon three pieces of data: (1) the vapor pressure of gasoline entering the blending unit, (2) the vapor pressure of butane entering the blending unit, and (3) the desired vapor pressure of the blended gasoline. Alternatively, or additionally, the process control unit can determine the blending ratio based upon (1) the vapor/liquid ratio of the gasoline, (2) the vapor pressure of butane entering the blending unit, and (3) the desired vapor/liquid ratio of the gasoline. In a preferred embodiment, the process control unit determines the blending ratio from the vapor pressure and vapor/liquid ratio assessments, adopting the lowest rate of butane blending from the two methods.
By blending gasoline and butane immediately before the gasoline is dispensed to a tanker truck, and by continuously controlling the ratio of gasoline and butane blended by the blending apparatus, a number of significant advantages are attained, including the following:
1. The amount of butane blended with the gasoline can be more thoroughly controlled, yielding less RVP variability among tanker truck shipments.
2. The butane and gasoline can be blended to yield consistent optimal performance of motor vehicles that employ the blended gasoline, regardless of the time of year in which the motor vehicle is operated, or the temperature or elevation at which such motor vehicle is operated.
3. The ratio of butane and-gasoline blended can be easily varied and controlled to comply with regional and/or seasonal RVP requirements imposed by EPA or state regulations upon-the sale of retail gasoline.
4. By continuously adding butane to gasoline dispensed to tanker trucks, and by continuously blending at the maximum RVP and vapor/liquid ratio allowable by law, tank farm operators are able to maximize the amounts of butane that they blend with gasoline, and minimize their cost basis for the gasoline sold.
The data required for the process control unit to properly blend butane and gasoline to prescribed conditions, especially fluid flow rates, can also be used to generate useful operational data. For example, by monitoring the rate at which butane is drawn from a butane storage tank, one is better able to predict when butane must next be purchased, and how much butane must be purchased, thereby ensuring better informed butane purchasing decisions. Moreover, by properly manipulating the data obtained from the blending process control unit; one is able to generate reports for gasoline sold from a particular tank farm as required by federal and state laws or regulations.